Within renewable energy, the collection of solar thermal technology can be found, which is of a technologically and economically importance in the domestic and industrial sector. Solar thermal energy produces electricity with a conventional thermoelectric cycle that requires heating a fluid at high temperature. These systems require the maximization of the concentration of solar energy at the point or points of absorption thereof, by using mirrors that can be completely flat, with a certain degree of spherical curvature, parabolic or cylinder-parabolic, depending on the technology of the solar thermal power plants.
Consequently, the value of the coefficient of reflectivity of the mirrors installed in these systems plays an important role in the performance of power plants that generate solar thermal energy. Furthermore, knowledge of these reflectivity values allows, together with information on environmental conditions of the area and other technical data of the plants, to forecast the power that will be generated in the near future in order for firms to properly manage energy resources.
For the operation and maintenance of electric energy production facilities, due to the large number of mirrors installed, it is convenient to have equipment that allows the characterization of reflectivity of each mirror quickly, conveniently and easily. The equipment that carries out a measurement of this type is called a reflectometer.
Given the optical characteristics of the solar energy absorbing elements which are included in these plants (maximum energy absorption and minimum energy losses, which determine dependencies of the optical parameters with the wavelength), the equipment must provide measurements of the mirrors according to the wavelength.
Similarly, the equipment must provide precise measurement of reflection value extremes close to the unit, generally in unfavorable environmental conditions, since the ambient light intensity will usually be high and even exceed in some cases, the signal to be measured itself. In addition, the requirement of high precision of the measurements is essential in solar thermal technology to maintain the efficiency in plants that produce electricity.
On the other hand, the reflection in the mirrors can be of two characters; diffuse and specular. Diffuse reflection is omnidirectional, unlike specular reflection in which the beam is reflected at a reflection angle equal to the angle of incidence. Due to the dirt that is deposited on the surface of the mirrors of the plant, the reflection of sunlight will have diffuse and specular components, specular reflection being useful only from the viewpoint of power generation, since it is the only one that will concentrate on the absorber element. Therefore, the equipment should minimize the contribution of diffuse reflection on the measurement of the reflection coefficient of the mirrors.
Finally, the equipment must be able to correctly measure the set of types of mirrors commonly used in the power plants. Specifically, it must be able to correctly measure flat mirrors, mirrors with a certain degree of spherical curvature, parabolic and cylinder-parabolic mirrors of different thicknesses without equipment adjustments.
A conventional reflectometer uses a broad spectrum light source and a variable filtering element that allows for sequentially select different wavelengths, such as a movable diffraction grating followed by a narrow slit. This option allows for varying the wavelength in a virtually continuous way, but in turn, results in a more complex and delicate system, with a low measurement dynamic range as the power of the input light that is achieved is very low. Furthermore, conventional equipment does not minimize the contribution of diffuse reflection, and in fact, in some cases it is of interest to collect all the scattered light and integrating spheres in detection are implemented.
The U.S. Pat. No. 5,815,254 describes a spectrophotometer device that can work in transmission measurement mode and reflection measurement mode. It uses a source of white light, halogen or Xe, optical fibers to carry the illumination light beam from the sample onto the sample surface, and a spectral analysis based on diffraction grating and a detector line.
The U.S. Pat. No. 3,862,804 describes double beam reflectometer equipment with switching mirror included in each measurement, the correction with the standard measurement, and integrating sphere to include in the measurement of scattered light reflection. The system uses white light, the monochromator for wavelength selection, illumination with collimated beams and integrating sphere in the detection which means that all the scattered light is collected and measured in the detection.
The U.S. Pat. No. 4,687,329 describes spectrophotometer equipment which uses a broad spectrum source, in this case ultraviolet, and various filters in fixed positions to perform a spectral measurement on a number of discrete points.
There is also a background of spectrophotometers in which a collection source of light sources of different wavelengths is used. In the patent US 2008/0144004 multiple light emitting diodes (LED) are used simultaneously to perform a transmission measurement for the detection of various analytes in blood. However, one true spectral measurement is not performed, but rather several simultaneous measurements at a few different wavelengths. Furthermore, there is no protection against ambient light nor is it possible to take measurements of reflection or reference.
None of the above equipment or other similar equipment meet the requirements necessary for measurement in the field of mirrors for solar collectors, either by range, sensitivity and/or mechanical configuration.